This invention relates to an improved cross current, moving bed-type, adsorption device for use in removing a specific component contained in a gas, such as the sulfur oxide contained in an exhaust gas.
In the case where a large volume of gas is treated with a solid adsorbent it is known that the cross current, moving bed-type, adsorption device is advantageous in the points of equipment area, controllability of the flow rate of the adsorbent and gas load, said device being constructed so that the gas flow is brought into contact with the adsorbent particle moving bed in a cross current manner. U.S. Pat. No. 3,716,969 issued to Isamu Maeda describes that a continuous moving layer adsorption device employed in an exhaust gas desulfurization system is made of an adsorption vessel as the main body filled with activated carbon and a rectifying device included therein, the adsorption vessel main body comprising a side wall provided with a number of louvers and located at the gas-introducing side of the vessel, another side wall having delivery holes located at the gas-exhausting side of the vessel, the distance between the gas-introducing side wall and the gas-exhausting side wall being larger toward the downward portion of the vessel, a hopper-like portion provided adjacently below the two side walls, and an elongated port provided at the bottom of the hopper-like portion for delivering the activated carbon; and the rectifying device comprising a rectifying body placed in the hopper-like portion and a rectifying plate extended downwardly from the rectifying body. According to this adsorption device, the center-dropping phenomenon and the suspension phenomenon of adsorbent particles can be prevented, said phenomena being apt to take place in the moving bed of the box-type moving bed adsorption device, thereby ensuring the uniform downward flow of adsorbent particles. In this connection, the center-dropping phenomenon is defined to be a phenomenon wherein the centrally located portion in the layers of the adsorbent particles just above the discharging port fall down prematurely, and the suspension phenomenon is defined to be a phenomenon wherein the adsorbent particles filled inside of the adsorption vessel is suspended in a crust-like configuration caused by a lateral compressive force, both of which are detailed in U.S. Pat. No. 3,716,969.
However, even such a device involves many troubles in order to achieve the uniform gas distribution. The reason is that in order to increase the gas treating capacity of the device it is inevitably necessary to increase the height of the adsorbent filled layer and consequently there is created a conspicuous difference in layer pressure between the particles located at the upper part and those located at the lower part thereof since the adsorbent filled bed is made to have a divergent structure, whereby the amount of the exhaust gas passing through the upper part is markedly increased. In addition, the increase in the height of the adsorbent particle layer brings about troubles such that the particles located at the bottom of the layer are liable to crush and wear by their own pressure and further attention must be paid to a probability that the uniform downward flow of particles is disturbed, although it is caused partly because the width of the bed is increased as a result of employing a large-scale equipment. Accordingly, it is natural that the device of U.S. Pat. No. 3,716,969 as it stands has a limitation in its capacity.
Another problem involved in the cross current, moving bed-type, adsorption device consists in that it is inferior in the coefficiency of utilization of adsorbent as compared with the counter current moving bed type device.
FIG. 1 is a graph illustrating one example of the changes in the amount of SO.sub.2 adsorbed on activated carbon with the passing of time measured by hanging a basket packed with the activated carbon in a SO.sub.2 -containing gas. As illustrated therein, the changes in the amount of SO.sub.2 adsorbed on the activated carbon with the passing of time can be divided into Zone I wherein the adsorbed amount increases in a linear, steep gradient, Zone II where the rate of increase of the adsorbed amount slows down and Zone III where the adsorbed amount reaches the saturation amount ultimately while increasing in a somewhat linear gradient. In order to maintain the effective utilization of the adsorbent, namely activated carbon, at a high level, it is preferable that the activated carbon should be utilized before and behind the boundary line between Zone II and Zone III rather than in Zone I. However, the relation between the residence time of activated carbon and the SOx concentration of the gas at the outlet port of the moving bed, as shown in FIG. 2, can be obtained from the practical treatment of the SOx-containing gas by means of a conventional cross current, moving bed, adsorption device using activated carbon as an adsorbent, which indicates a tendency that the SOx concentration of the gas at the outlet port decreases as the residence time of the activated carbon is shortened and increases as the residence time is prolonged. It may be summarized that, in the case of said conventional cross current, moving bed, adsorption device, the activated carbon must be used in the zone where the adsorption rate does not slow down, that is, Zone I illustrated in FIG. 1 in order to maintain the desulfurization rate at a high level, because a prolonged residence time of the activated carbon slows down the adsorption rate of SOx onto the activated carbon, thereby causing the SOx concentration of the gas at the outlet port to increase. However, this is not desirable because the utilization of the activated carbon can not performed effectively.
In order to maintain the desulfurization rate at a high level without spoiling the effective utilization of the activated carbon, there is the necessity of providing the cross current, moving bed with a means capable of compensating for the slowing down of the adsorption rate when the residence time of the activated carbon is prolonged. Specifically it is necessary to design the cross current, moving bed so that the space velocity of the gas in the lower region of the cross current, moving bed may be smaller than that in the upper region thereof.
In the continuous moving layer-type, adsorption device disclosed in U.S. Pat. No. 3,716,969, wherein the moving bed has divergent structure, the space velocity of the gas passing through the moving bed in a cross current slows down as the bottom of the moving bed is approached. Accordingly, the cross current, moving bed of this type may be said to compensate for the aforesaid slowing down of the adsorption rate in a way. However, this cross current, moving bed is defective in that it is unsuitable for the purpose of treating a large capacity of gas because the layer height (H in FIG. 2) can not be increased without elevating the pressure in masses of adsorbent particles per se.
Furthermore, Japanese Patent Publication No. 46790/1978 teaches a moving bed in which the flow rate of gas in both the upper and lower regions of the moving bed is controlled by the provision of a baffle plate, thereby lowering the space velocity of the gas passing through the lower region thereof, but this moving bed also has the problem that when the layer height is increased, the pressure in masses of adsorbent particles is elevated to an excessive degree.